The invention relates to a pressure sensitive switch disposed inside the seat of a vehicle for detecting whether there is a passenger on the seat or not, and a detector having the pressure sensitive switch.
As vehicles having high-level functions increase, various pressure sensitive switches are used to control such functions as an air bag system or an indication of a seat-belt-wear sign by detecting a seated passenger and his/her weight.
A conventional pressure sensitive switch will be described with reference to FIG. 8 through FIG. 11
FIG. 8 is a cross sectional view of the conventional pressure sensitive switch. Movable contact layer 2, which is a pressure sensitive resistance element and made of resins-like epoxy resin, phenol resin, or polyester resin-with conductive powder like carbon dispersed therein, is formed by printing underneath top sheet 1 made of flexible insulating films like polyethylene terephthalate or polyimide. Bottom sheet 3 made of flexible insulating film-like polyethylene terephthalate or polyimide-is disposed under top sheet 1. Plural pairs of comb-like conductive layers 4A and 4B are formed by etching a copper foil bonded onto bottom sheet 3 or by printing flexible resin-like polyester or epoxy resin-with silver or carbon dispersed therein over bottom sheet 3. Fixed-contact layers 5A and 5B, which are pressure sensitive resistance elements and made of resin-like epoxy resin, phenol, or polyester-with carbon dispersed therein is formed by printing over conductive layers 4A and 4B such that layers 5A and 5B are opposed to layer 2 over top sheet 1.
Insulating spacer layer 6 made of resins like epoxy resin, phenol, or polyester is formed by printing over bottom sheet 3 so as to expose fixed-contact layers 5A and 5B. Adhesive layer 7 made of resins like polyester or acrylic resin is formed by printing underneath top sheet 1 so as to expose movable contact layer 2. Adhesive layer 7 bonds top sheet 1 and bottom sheet 3 together and holds a predetermined distance between movable contact layer 2 and fixed-contact layers 5A and 5B.
The pressure sensitive switch with a configuration described above is disposed inside the seat of a vehicle. When a passenger sits on the seat, top sheet 1 is dented by the weight of the passenger and movable contact layer 2 underneath top sheet 1 makes contact with fixed-contact layers 5A and 5B over bottom sheet 3. Then, as a load placed on the seat increase, a contacting area among particles of conduct powder dispersed in movable contact layer and fixed-contact layers 5A and 5B increases, a resistance in the pressure-sensitive-resistance element decreases. When the passenger is seated completely and the load placed on the seat reaches a constant value, the contacting area among particles of conductive powder provides a certain value. A resistance becomes a constant value which is not greater than a predetermined value as compared with starting to contact. The sum of resistances in each point in movable contact layer 2 and fixed contact layers 5A and 5B is detected as a resistance of the pressure sensitive switch.
A conventional seating detector having the above pressure sensitive switch will be described below.
FIG. 10 is a circuit diagram of the conventional seating detector. Pressure sensitive switch 8 with the configuration as described above is coupled to control circuit 9. One of conductive layers 4A and 4B in pressure sensitive switch 8 is coupled to a power source, and the other conductive layer is coupled to fixed resistor 10. Branch point A10 is coupled to a negative input of comparator 12 which determines a predetermined threshold voltage and controls an output through fixed resistor 11. Fixed resistors 13 and 14 for setting the threshold voltage are coupled to a positive input of comparator 12 through fixed resistor 15 from branch point B10. Fixed resistor 16 is coupled to comparator 12 as a feedback resistance, and output section 17 of comparator 12 is coupled to the power source through fixed resistor 18.
Actuation circuit 20 is coupled to output section 17 of comparator 12 and controls an operation of an air bag system or indicating a seat-belt-wear sign.
In the seating detector with the configuration described above, when the seating detector is actuated, the predetermined threshold voltage adjusted by fixed resistors 13 and 14 is fed to the positive input of comparator 12 through fixed resistor 15 from branch point B10. When a passenger is seated and the resistance of pressure sensitive switch 8 becomes low, the voltage of branch point A10 determined by pressure sensitive switch 8 and fixed resistor 10 becomes high. After the passenger seated completely, the voltage determined by pressure sensitive switch 8 and fixed resistor 10 becomes constant. The constant voltage is fed to the negative input of comparator 12 through fixed resistor 11 from branch point A10. Comparator 12 compares the threshold voltage from branch point B10 and the voltage from branch point A10. When the voltage from branch point A10 is higher than the threshold voltage from branch point B10, the resistance is not greater than a predetermined value because a load of not less than a predetermined value is placed on the pressure sensitive switch. When the seating detector determines that the passenger is seated completely and the weight of the passenger is not less than a predetermined value, control signals is output to actuation circuit 20 from output section 17.
In pressure sensitive switch 8 used as described above, movable contact layer 2 and fixed-contact layers 5A and 5B as pressure sensitive resistance elements are formed by screen printing with ink made of resin having conductive powder dispersed therein. A resistance characteristic varies according to a compounding ratio of conductive powder and resin in the ink used for printing, variation in the diameter of particle of conductive powder, thickness of a printing plate, or a drying condition in printing. As shown in the resistance characteristic chart in FIG. 11, for example, a load placed on the switch is inversely proportional to a resistance in reference characteristic (c). When the conductive powder is contained in movable contact layer 2 and fixed contact layers 5A and 5B is small, and the printed layers are thin, the whole resistance is high as shown in characteristic (d). In contrast, when much conductive powder is contained and the printed layers are thick, the whole resistance is low as shown in characteristic (e).
For testing resistance characteristics of the pressure sensitive switch, loads F1 and F2 are placed on the switch respectively on a basis of a reference resistance R0 at reference load F0. The test determines whether or not these resistance are within tolerances of R1 and R2 respectively, and then, determines the quality of the pressure sensitive switch. For the characteristic (d) with a high resistance, when load F2 is placed on the switch, the resistance is in a range of R1 to R2. However, when load F1 is placed on the switch, the resistance becomes R3, which is out of the tolerance higher than R2. Therefore, this pressure sensitive switch is out of the reference. For characteristic (e) with low resistance, when load F1 is placed on the switch, the resistance is in a range of R1 to R2. However, when load F2 is placed on the switch, the resistance becomes R4, which is out of the tolerance lower than R1. The pressure sensitive switch is therefore determined that it is out of the reference.
The pressure sensitive switch determined to be out of the reference cannot be used, therefore, a lot of loss occurs in actual production.
An inexpensive pressure sensitive switch by reducing the loss which occurs during the production of the switch and a seating detector having the pressure sensitive switch are presented. The pressure sensitive switch indicates a mark on a top or bottom sheet to show a resistance characteristic of contact layers in the switch. At least one of a wiring layer and a resistor layer, which has a different conducting condition and different resistance, is formed corresponding to each resistance characteristic. The inexpensive pressure sensitive switch whose resistance characteristic can be determined and the seating detector having the pressure sensitive switch are provided.